Shrinkable thermoplastic films have found many applications in packaging of meats, cheeses, poultry, seafood and numerous other food and non-food products. For packaging some foodstuffs, for instance meat and some cheeses, the film should include a layer that is a barrier to the passage of gases, particularly oxygen. For packaging other foodstuffs, for instance poultry and some other cheeses, and also for packaging non-food materials, no such barrier layer is required.
There is always the search for improvement in these films to give them better abuse resistance, better tear resistance, improved clarity, easier handling and better barrier properties. One film of this type is a multi-layer film having layers of polyethylene/saran/polyethylene which is disclosed in U.S. Pat. No. 3,821,182 which issued on Jun. 28, 1974 to William G. Baird, Jr. et al. The shrink and abuse resistance of such a film is improved by irradiating the film to cross-link the polyethylene layers prior to heating and orienting the film by the trapped bubble technique.
U.S. Pat. No. 3,741,253, which issued on Jun. 26, 1973 to Harri J. Brax et al, discloses a multi-ply laminate which has a first layer of cross-linked ethylene-vinyl acetate copolymer directly joined to a middle layer of a copolymer of vinylidene chloride which is joined to another ethylene-vinyl acetate copolymer layer. The ethylene-vinyl acetate copolymer (hereinafter EVA) layer has improved properties over the previously used polyethylene and, in the extrusion coating method used to produce the multi-layer film according to the Brax et al patent, the substrate EVA layer is preferably cross-linked by irradiation before the saran layer is extrusion coated thereon, thus avoiding irradiation of the saran layer. Saran (vinylidene chloride homo- or copolymer) tends to discolor under high energy irradiation.
An alternative and successful multi-layer film where a hydrolysed ethylene-vinyl acetate copolymer is used as a barrier layer instead of saran is disclosed in U.S. Pat. No. 4,064,296 which issued on Dec. 29, 1977, to Normal D. Bornstein et al. A heat shrinkable multi-layer film is formed by coextruding the hydrolysed ethylene-vinyl acetate copolymer (sometimes abbreviated "HEVA" or called ethylene-vinyl alcohol and abbreviated "EVAL" or "EVOH".) Since EVOH does not suffer from the effects of radiation a coextruded product such as EVA/EVOH/EVA can readily be cross-linked by irradiation before orientation.
Another way of improving the performance of packaging films has been to blend various polymers. U.S. Pat. No. 3,090,770, which issued on May 21, 1973 to Razmic S. Gregorian, discloses the blending of cross-linked polyethylene with non-cross-linked polyethylene to improve the clarity of a film. Such blends use differing proportions of high, low and medium density polyethylene. This patent also discloses a cross-linked polyethylene; and, U.S. Pat. No. 3,118,866, which issued on Jan. 28, 1964 to the same inventor, is directed to an ethylene composition and the process of cross-linking by chemical means. The olefin polymers and copolymers have been particularly attractive because of low cost, availability, and wide range of satisfactory characteristics for packaging films.
Recently, medium and low density linear polyethylenes have become commercially available and have begun to be used in a number of packaging applications. One early patent in this field is U.S. Pat. No. 4,076,698, which issued on Feb. 28, 1978 to Arthur William Anderson and discloses an interpolymer composed of ethylene and mono-alpha-olefinic hydrocarbons containing five to ten carbon atoms per molecule, the proportion of the mono-olefinic hydrocarbon being 3 to 7 percent of the weight of the interpolymer, with a melt index from 0.3 to 20 and a density of 0.93 to 0.94 g/cc. Linear polymers of this type are characterized by actually being an interpolymer or copolymer with another olefin and having a relatively straight molecular chain, that is, having a chain with no side branches or limited side branching. Low density versions of this type of film, where density is in the range of 0.920 to.0.926, are produced by a low pressure process, as opposed to the high pressure process which produces a branched, low density polyethylene. Linear low density polyethylene, sometimes abbreviated hereinafter as "LLDPE", has found many applications and uses as exemplified by U.S. Pat. No. 4,364,981 which issued on Dec. 21, 1982 to Jerome T. Horner and discloses an EVA/LLDPE/EVA, structure as does also U.S. Pat. No. 4,399,180 which issued on Aug. 16, 1983 to William F. Briggs et al. In U.S. Pat. No. 4,457,960 a multi-layer structure is disclosed of EVA/Saran/EVA-LLPDE-blend.
Still another polymeric material has more recently entered the market having different properties from the copolymers which comprise the LLDPE class of materials. These copolymers are known as very low density polyethylene (hereinafter abbreviated "VLDPE"). Whereas conventional polyethylenes and LLDPE have densities as low as 0.912, the VLDPE currently on the market have densities below 0.910, specifically down to about 0.860. European Published Patent Application No. 120,503 (Union Carbide), published Oct. 3, 1984, discloses a method of making VLDPE. In "Plastics Technology" magazine for September 1984 at page 113, a news item entitled "Introducing Very Low Density PE" briefly described some of VLDPE properties and stated that it's what the manufacturer ". . . calls an entirely new class of polyethylene, consisting of linear copolymers that can be produced at densities down to 0.89 or lower. What makes them special is a unique combination of properties in between those of standard PE's and polyolefinic rubbers". In the October 1984 issue of "Plastics Technology" at page 13 another article appeared entitled "New Kind of Polyethylene Combines Flexibility, Toughness, Heat Resistance". This article lists a number of the properties of VLDPE and compares them with ethylene-vinylacetate (EVA) and states that uses for this material is for squeeze tubes, bottles, hoses, tubing, drum liners and film. VLDPE is also listed as having potential as an additive. It is expected to be used as a blending resin in high density polyethylene, polypropylene, EVA, and some ethylene-propylene rubbers (EPR), with all of which VLDPE is compatible. According to the article, the first two commercially available grades are from Union Carbide. One resin, designated "DFDA-1138 NT7", has a narrow molecular weight distribution, higher toughness, clarity, and gloss and FDA clearance for food contact. The other resin is DFDA-1138 which is aimed particularly at film, has a broad molecular weight distribution, and is superior in processability. On page 15 in the same article, it is stated that "the new resins have been injection molded, extruded, blow molded, and thermoformed on standard equipment". It is noted that blown film can be extruded on systems designed either for conventional LDPE or LLDPE. However, the company generally recommends LLDPE-type screw designs in higher torque capability, especially with narrow-MWD grades. The article observes that the enlarged die gaps required by LLDPE are not required for VLDPE and that conventional blown film die gaps of 30-40 mil have proven satisfactory at blow up ratios of 2-3:1. For blown film, DFDA1137 and 1138 are said to extrude much like 2-Melt Index LLDPE or 0.5-Melt Index LDPE. An article similar to the one in "Plastics Technology" appeared in the October 1984 issue of "Plastics World" at page 86.
In the above mentioned European Patent Application a process for preparing very low density ethylene polymers in a fluidized bed is described. These ethylene polymers are classified as having a density of less than 0.91 and having a melt flow index which is preferably from 0.2 to 4.0.
The incorporation into heat shrinkable films of conventional ethylene/alpha-olefins produced by Ziegler-Natta catalyst systems is well known. Ziegler-Natta catalystic methods are commonly used throughout the polymer industry and have a long history tracing back to about 1957.
These systems are often referred to as heterogeneous since they are composed of many types of catalytic species each at different metal oxidation states and different coordination environments with ligands. Examples of Ziegler-Natta heterogeneous systems include metal halides activated by an organometallic co-catalyst, such as titanium or magnesium chlorides complexed to trialkyl aluminum and may be found in patents such as U.S. Pat. Nos. 4,302,565 and 4,302,566. Because these systems contain more than one catalytic species, they possess polymerization sites with different activities and varying abilities to incorporate comonomer into a polymer chain.
The result of such multi-site chemistry is a product with poor control of the polymer chain architecture both within the sequence of a single chain, as well as when compared to a neighbouring chain. In addition, differences in catalyst efficiency produce high molecular weight polymer at some sites and low molecular weight at others. Therefore, copolymers produced using these systems lead to polymer products which are mixtures of chains some high in comonomer and other with almost none. For example, conventional Ziegler-Natta multi-site catalysts may yield a linear ethylene/alpha-olefin copolymer having a mean comonomer percentage of 10, but with a range of 0% to 40% comonomer in individual chains. This, together with the diversity of chain lengths results in a tryl heterogeneous mixture also having a broad molecular weight distribution (MWD).
Linear low density polyethylene (LLDPE) has enjoyed great success as a raw material choice for packaging films. The term LLDPE is generally understood to describe copolymers of ethylene and one or more other alpha olefin monomers which are polymerized at low pressure using a Ziegler-Natta catalyst to achieve a density range of about 0.915 to about 0.940. Although no clear standard exists, LLDPE polymers are often marketed in subgroups of densities such as linear medium density (LMDPE), linear low density polyethylene, linear very low density (VLDPE), or linear ultra low density polyethylene (ULDPE). These classifications are for marketing use and will vary by supplier.
These materials are different from high pressure low density polyethylene (LDPE) which is generally understood in the trade as a highly branched homopolymer having a single low melting point. For example, a 0.92 density LDPE would typically have a melting point at about 112.degree. C. while a corresponding density LLDPE would have melting point at 107.degree., 120.degree., and 125.degree. C. The multiple melting points are commonly observed with LLDPE and are a consequence of the above mentioned heterogeneous incorporation of comonomer.
Recently a new type of ethylene copolymer has been introduced which is the result of a new catalyst technology. Examples of introductory journal articles include "Exxon Cites `Breakthrough` in Olefins Polymerization," Modern Plastics, July 1991, p.61; "Polyolefins Gain Higher Performance from New Catalyst Technologies," Modern Plastics, October 1991, p.46; "PW Technology Watch," Plastics World, November 1991, p. 29; and "," Plastics Technology, November 1991, p. 15.
These new resins are produced using metallocene catalyst systems, the uniqueness of which resides in the steric and electronic equivalence of each catalyst position. Metallocene catalysts are characterized as having a single, stable chemical type rather than a volatile mixture of states as discussed for conventional Ziegler-Natta. This results in a system composed of catalyst positions which have a singular activity and selectivity. For this reason, metallocene catalyst systems are often referred to as "single site" owing to the homogeneous nature of them, and polymers and copolymers produced from them are often referred to as single site resins by their suppliers.
Generally speaking, metallocene catalysts are organometallic compounds containing one or more cyclopentadienyl ligands attached to metals such as hafnium, titanium, vanadium, or zirconium. A co-catalyst, such as but not limited to, oligomeric methyl alumoxane is often used to promote the catalytic activity. By varying the metal component and the cylopentadienyl ligand a diversity of polymer products may be tailored having molecular weights ranging from about 200 to greater than 1,000,000 and molecular weight distributions from 1.5 to about 15. The choice of co-catalyst influences the efficiency and thus the production rate, yield, and cost.
Exxon Chemical, in U.S. Pat. No. 4,701,432 sets out examples of which olefin catalyst systems are of the metallocene class and which are non-metallocene. The cite bis(cyclopentadienyl) dichloro-transition metal, bis(cyclopentadienyl) methyl, chloro-transition metal, and bis(cyclopentadienyl) dimethyl-transition metal as examples of metallocene catalysts, where the metals include choices such as titanium, zirconium, hafnium, and vanadium. The patent further provides examples of non-metallocene catalysts as being TiCl.sub.4, TiBr.sub.4, Ti(0C.sub.4 H.sub.9).sub.2 Cl.sub.2, VCl.sub.4, and VOCl.sub.3.
Similarly, C. P. Cheng, at SPO 91, the Specialty Polyolefins Conference sponsored by Schotland and held in Houston, Tex. in 1991, cited TiCl.sub.3 /AlR.sub.2 Cl and MgCl.sub.2 /TiCl.sub.4 /AlR.sub.3 as examples of non-metallocene Ziegler-Natta catalysts and transitions metal cyclopentadienyl complexes as examples of metallocene homogeneous polyolefin catalysts.
As a consequence of the single site system afforded by metallocenes, ethylene/alpha-olefin copolymer resins can be produced with each polymer chain having virtually the same architecture. Therefore, the copolymer chains produced from single site systems are uniform not only in chain length, but also in average comonomer content, and even regularity of comonomer spacing, or incorporation along the chain.
In contrast to the above mentioned Ziegler-Natta polymers, these single site metallocene polymers are characterized as having a narrow MWD and narrow compositional distribution (CD). While conventional polymers have MWD's of about 3.5 to 8.0, metallocenes range in MWD from about 1.5 to about 2.5 and most typically about 2.0. MWD refers to the breadth of the distribution of molecular weights of the polymer chains, and is a value which is obtained by dividing the number-average molecular weight into the weight-average molecular weight. The low CD, or regularity of side branches chains along a single chain and its parity in the distribution and length of all other chains, greatly reduces the low MW and high MW "tails". These features reduce the extractables which a rise from poor LMW control as well as improve the optics by removing the linear, ethylene-rich portions which are present in conventional heterogeneous resins.
Thus, conventional Ziegler-Natta systems produce heterogeneous resins which reflect the differential character of their multiple catalyst sites while metallocene systems yield homogeneous resins which, in turn, reflect the character of their single catalytic site.
Another distinguishing property of single site catalyzed ethylene copolymers is manifested in their melting point range. The narrow CD of metallocenes produces a narrow melting point range as well as a lower Differential Scanning Calorimeter (DSC) peak melting point peak. Unlike conventional resins which retain a high melting point over a wide density range, metallocene resin melting point is directly related to density. For example, an ethylene/butene copolymer having a density of 0.905 g/cc produced using a metallocene catalyst has a peak melting point of about 100.degree. C., while a slightly lower density ethylene/butene copolymer which was made using a conventional Ziegler catalyst reflects its heterogeneous nature with a melting point at about 120.degree. C. DSC shows that the Ziegler resin is associated with a much wider melting point range and actually melts higher despite its lower density.
While providing improved physical properties such as optics, low extractables and improved impact, the narrow compositional distribution of some typical metallocene catalyzed resins can cause some processing difficulties. It has been found that such processing problems are avoided if some limited long chain branching is introduced. That is, a typical metallocene catalyzed ethylene alpha-olefin may be thought of as a collection of linear chains each of substantially identical length, each having approximately the same number of short chain (comonomer) branches distributed at regular intervals along that length. Splicing an abbreviated linear chain with the same regular comonomer distribution onto each of the linear chains, or at least some of the chains in the collection, yields an ethylene alpha-olefin with essentially all of the physical properties of the original copolymer, but which an improved "body" or melt strength for improved processability including improved extrudability, orientation speeds and susceptibility to irradiation.
In recent years several resin suppliers have been researching and developing metallocene catalyst technology. The following brief discussion should be viewed as representative rather than exhaustive of this active area of the patent literature.
Dow in EP 416,815 disclosed the preparation of ethylene/olefin copolymers using monocyclopentadienylsilane complexed to a transition metal. The homogenous ethylene copolymers which may be prepared using this catalyst are said to have better optical properties than typical ethylene polymers and be well suited for film or injection molding.
As will be shown below, it has been found that resins produced by the Dow process exhibit improved physical properties characteristic of single site catalyzed resins but also possess a processability similar to that of conventional Ziegler-Natta copolymers. It is believed that the Dow metallocene resins possess the limited long chain branching discussed above.
Welborn in Exxon U.S. Pat. No. 4,306,041 discloses the use of metallocene catalysts to produce ethylene copolymers which have narrow molecular weight distributions.
Chang, in Exxon U.S. Pat. No. 5,088,228 discloses the production of ethylene copolymers of 1-propene, 1-butene, 1-hexane, and 1-octene using metallocene catalysts.
Exxon in U.S. Pat. No. 4,935,397 discloses the production of ethylene copolymers using metallocene catalysts to manufacture polymer suitable for injection molding or thermoforming.
Welborn, in Exxon U.S. Pat. No. 5,084,534 discloses the use of bis(n-butylcyclopentadienyl) zirconiumdichloride to produce high molecular weight polyethylene having a polydispersity of 1.8 and a density of 0.955 g/cc.
In Exxon U.S. Pat. No. 3,161,629 a cyclopentadienyl complex is disclosed which may be used to produce polyolefins having controlled molecular weight and density suitable for use in extrusion or injection molding.
Canich in Exxon U.S. Pat. Nos. 5,055,438 and 5,057,475 discloses the use of mono-cyclopentadienyl catalysts having a unique silicon bridge which may be employed to select the stereo-chemical structure of the polymer. Catalysts such as methyl, phenyl, silyl, tetramethylcyclopentadienyl-tertbutylamido zirconium dichloride may be used to produce polyethylene and ethylene copolymers suitable for films and fibers.
Mitsui Toatsu in JP 63/175004 employed bis(cyclopentadienyl) ethoxy-ZrCl to prepare homogenous ethylene copolymers.
Mitsubishi in JP 1,101,315 discloses the use of bis (cyclopentadienyl)ZrCl.sub.2 for the preparation of ethylene butene copolymers.
It should be noted that at least some previously available ethylene based linear polymers approximated the physical and compositional properties achieved by the present metallocene catalyzed polyolefins. For example, in "Sequence and Branching Distribution of Ethylene/1-Butene Copolymers Prepared with a Soluble Vanadium Based Ziegler-Natta Catalyst," Macromolecules, 1992, 25, 2820-2827, it was confirmed that a soluble vanadium based Ziegler-Natta catalytic system VOCl.sub.3 /Al.sub.2 (C.sub.2 H.sub.5).sub.3 Cl.sub.3, acts essentially as a single site catalyst although VOCl.sub.3 is not a metallocene. Homogeneous copolymers produced by such a catalyst system have been commercially available for several years. An example of such are the resins sold under the trade-mark Tafmer(TM) by Mitsui.
U.S. Pat. No. 4,501,634 to Yoshimura et al is directed to an oriented, multilayered film which includes a Tafmer as a blend component in at least one layer.
Japanese Kokoku 37307/83 to Gunze Limited was directed to a heat-sealable biaxially oriented composite film wherein the heat seal layer contains Tafmer in a blend.
The foregoing patents disclose homogeneous ethylene alpha-olefins having densities below 0.90 g/cc.
A successful and useful film is made according to the process shown in U.S. Pat. No. 3,741,253 mentioned above. A heat shrinkable bag can be made from such film which has wide application, particularly for meat, poultry, and some dairy products. Heat shrinkable polymeric films have gained widespread acceptance for packaging meat, particularly fresh meat and processed meat. Bags made from the heat shrinkable film are sealed at one end with the other end open and ready to receive a meat product. After the cut of meat is placed in the bag, the bag will normally be evacuated and the open end of the bag closed by heat sealing or by applying a clip, e.g., of metal. This process is advantageously carried out within a vacuum chamber where the evacuation and application of the clip or heat seal is done automatically. After the bag is removed from the chamber it is heat shrunk by applying heat. This can be done, for instance, by immersing the filled bag into a hot water bath or conveying it through a hot water shower or a hot air tunnel, or by infra red radiation.
In the usual distribution chain, a whole primal or sub-primal is packaged within shrink bags of this type. The meat within the bag will travel from a central slaughterhouse where it has been packaged to a retail supermarket where the bag will be opened and the meat will be cut for retail portions. Thus, the bags of this type must satisfy a number of requirements which are imposed by both the slaughterhouse or packing house and by the bag user. Furthermore, often the bag is placed in the showcase at the retail supermarket for special promotions when a whole loin, for example, is to be sold to a consumer. For retail use, particularly, it is desirable to have an attractive package. This requires relatively complete shrinkage of the bag around the product, so that the bag is not wrinkled and blood and juices are not trapped in the folds of the wrinkles.
Another important characteristic of a bag is the capability of the bag to physically survive the process of being filled, evacuated, sealed, closed, heat shrunk, boxed, shipped about the country, unloaded, and stored at the retail supermarket. This type of abuse rules out many polymeric films.
Another feature required by bags used for the foregoing described application is that the bag must also be strong enough to survive the handling involved in moving packaged meat which may weigh 100 pounds or more or large chunks of cheese weighing 60 lbs. or more. In particular, when the chunk of meat or cube of cheese is pushed into the bag its bottom seal must withstand the force of the meat or cheese as it hits the seal. Also, in bags that are made by folding a sheet with the fold as the bottom of the bag and by sealing the sides, seal strength is an important factor.
One of the more common hazards in packaging and distributing products in flexible packaging materials is the hazard of the material receiving a puncture which will release the vacuum inside the bag and allow oxygen to enter. Anything from the application of the clip to the presence of a bone in the meat can cause a puncture.
Canadian Patent Application Serial No. 502,615 of Ferguson et al discloses multi-layer thermoplastic barrier film comprising:
(a) a layer comprising very low density polyethylene having a density of less than 0.910 g/cc, PA1 (b) a barrier layer comprising a material selected from the group consisting of: 1) copolymers of vinylidene chloride and 2) hydrolyzed ethylene-vinyl acetate copolymers; and PA1 (c) a thermoplastic polymer layer, said layer being on the side of the barrier layer opposite to that of layer (a); PA1 (i) a layer composed of an ethylene-vinyl acetate copolymer or a linear ethylene-alpha-olefin copolymer or a blend of an ethylene-vinyl acetate copolymer and a linear ethylene-alpha-olefin copolymer; and PA1 (ii) a layer composed of a blend of (a) a linear ethylene-alpha-olefin copolymer; (b) a material selected from the group consisting of ethylene-vinyl acetate copolymers, ethylene-butyl acrylate copolymers, ethylene-methyl acrylate copolymers, ethylene-methyl methacrylate copolymers, ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymer and ethylene-carbon monoxide copolymers; and (c) a narrow molecular weight linear ethylene-alpha-olefin copolymer having a density of less than 0.9 g/cc, preferably 0.870 to less than 0.900 g/cc, preferably 0.870 to 0.885 g/cc. PA1 a) a film wherein layer (i) is a blend of an ethylene-vinyl acetate copolymer and a linear ethylene-alpha-olefin copolymer; PA1 b) a film wherein the linear ethylene-alpha-olefin copolymer present in layer (i) has a density of about 0.920 g/cc; PA1 c) a film wherein layer (ii) has a substantially greater thickness than layer (i); PA1 d) a film which comprises a further layer of material resistant to oxygen transmission; this material may be a copolymer of vinylidene chloride, especially a copolymer of vinylidene chloride with vinylchloride or methyl acrylate. Alternatively the material resistant to oxygen transmission is a copolymer of ethylene-vinyl acetate in which the acetate-moieties have been partially or completely hydrolyzed. This may be mixed with the vinylidene chloride copolymer. If the material resistant to oxygen transmission is a copolymer of ethylene-vinyl acetate in which the acetate moieties have been partially or completely hydrolyzed to give ethylene-vinyl alcohol copolymer then adhesive layers will be required to make this layer adhere to adjacent layers thereby giving a film with a further two (adhesive) layers. Typical adhesives include acrylic acid modified ethylene-vinyl acetate, anhydride modified ethylene-vinyl acetate and methacrylate resins; PA1 e) a film which comprises a further layer of a thermoplastic polymeric material. The further layer may comprise a copolymer of ethylene-vinyl acetate; PA1 f) a film which comprises; PA1 Sealant layer: blend of 90% EVA with 6% VA content and 10% linear ethylene-alpha-olefin copolymer of density 0.920 g/cc (Dowlex* 2045, available from Dow Chemical Company). FNT *Trade-Mark PA1 Substrate (core) layer: blend of 80% of linear ethylene-alpha-olefin copolymer of density 0.905 g/cc (Attane* 4203 available from Dow Chemical Company) and 20% of EVA with 18% VA content. FNT *Trade-Mark PA1 Barrier layer: 96% of a copolymer composed of 91.5% vinylidene chloride and 8.5% methyl acrylate, and 4% of epoxidized soya bean oil plasticizer, plus Irganox* 1010 antioxidant. FNT *Trade-Mark PA1 Evaluation of Processing Conditions--viz. Temperature Profile, Back Pressure in extruders, Cooling, Motive Load, Rates & Yields. PA1 Evaluation of Desired Physical Properties in different blends and thicknesses. PA1 Final Assessment & Pilot Plant Run. PA1 (a) "Constrained Geometry Catalyst Technology" resin (CGCT) available from Dow; PA1 (b) "Single-Site" Catalyzed (metallocene catalyst) resins (SSC) available from Exxon.
the multi-layer film being oriented and heat shrinkable at a temperature below 100.degree. C. This film hag been used to make heat-shrinkable bags to contain meat, cheese, and the like. A commercial product, within the scope of this patent application, that has met with success is in fact composed of four layers. An inner layer is formed from a blend of 90% of an EVA copolymer containing 6% vinyl acetate and 10% of an ethylene-alpha-olefin copolymer of density 0.912 g/cc. A second layer is composed of a blend of 80% of linear ethylene-alpha-olefin copolymer of density 0.912 g/cc and 20% of an EVA copolymer containing 20% vinyl acetate. A third, barrier layer is composed of a copolymer of vinylidene chloride. A fourth, outer layer is composed of a blend of 91% of an EVA copolymer containing 9% vinyl acetate and 9% of linear ethylene-alpha-olefin copolymer of density 0.912 g/cc. This film is prepared by co-extruding the two inner layers to form a tape of circular cross-section, irradiating to cause cross-linking, coextruding the barrier layer and the outer layer onto the outside wall of the tape, biaxially stretching the product, cutting it into lengths and heat-sealing each length at one end to form a heat-sealable, heat-shrinkable bag. The four layer wall of the bag has a thickness of about 2.4 mil. PA2 (i) a layer composed of a blend of ethylene-vinyl acetate copolymer and a linear ethylene-alpha-olefin copolymer preferably having a density of below about 0.920 g/cc; PA2 (ii) a layer composed of (a) a linear ethylene-alpha-olefin copolymer preferably butene, hexene or octene; (b) a material selected from the group consisting of ethylene-vinyl acetate copolymers and ethylene-n-butyl acrylate copolymers; and (c) a narrow molecular weight linear ethylene-alpha-olefin copolymer having a density of less than 0.900 g/cc; PA2 (iii) a layer composed of a vinylidene chloride copolymer or an ethylene-vinyl acetate copolymer in which the acetate moieties have been partially or completely hydrolyzed; and PA2 (iv) a layer composed of a copolymer of ethylene-vinyl acetate or a blend of ethylene-vinyl acetate copolymer and ethylene-alpha-olefin copolymer preferably in the proportion 91%:9% by weight, particularly 92.5%:7.25% by weight. The ethylene-alpha-olefin copolymer commonly will have a density of greater than 0.915 g/cc.
Although this commercial product works well, there are some difficult applications for which this product could be improved upon, for instance packaging picnic hams, and improvements are still sought in the areas of abuse resistance and shrinkage. Abuse is the term used to describe the treatment that a bag is subjected to when it is packed in a high speed packing operation for instance in a meat packing plant. A bag must withstand the impact of the meat entering the bag, without that causing any breakage in the heat seal at the initially closed end of the bag. If the meat has projecting bone the bag must withstand the impact of the bone without puncturing. The bag when evacuated and sealed must with stand hydrostatic pressures of blood and juices from the meat.
One approach to improving abuse resistance is to increase the thickness of the laminate film. It has been expected that this approach would lead to improvement in abuse resistance only at the expense of deterioration in other important properties, and would therefore be unacceptable. An increase in the thickness leads to a reduction of the elasticity of the film which results in increased stiffness of the film. This increased stiffness leads to formation of creases and increases the risk of crease fractures, resulting in increased leakage in handling. Another disadvantage is reduced heat shrinkage. It is desirable that the film shall have high heat shrinkage for several reasons. Film with high heat shrinkage encloses the packed foodstuff more closely to yield a packed product with greater aesthetic appeal, which is particularly important at the retail level. Also, high shrinkage reduces the formation of ears in the package. Regions of a sealed and heat shrunk package that are not separated by the packed material are referred to as ears, that is regions where the two inner surfaces of the bag are in contact with each other. Ears are unsightly, and for this reason should be as small as possible. Also ears project and with large ears there is increased risk that ears will catch on projections encountered during handling and be torn, resulting in a leaking pack.
There has now surprisingly been found a film composition with enhanced tensile strength, interply adhesion and resistance to tear propagation.